Soft reboot causes issues with debugger is stopped

[josesimoes]

Details about Problem

Target: all

Firmware image version:

Worked before? If so, with which nanoFramework image version:

Description

There is a race condition with g_CLR_DBG_Debugger structure which is is free’d at a soft reboot https://github.com/nanoframework/nf-interpreter/blob/136a95eca192cf9ab881da1840025c855ab91551/src/CLR/Debugger/Debugger.cpp#L125 and then, in the short time interval before it is re-initialized, a message from the extension would occasionally arrive and use this now invalid structure. This leds to a hard-fault.

A good fix for this is to set the g_CLR_DBG_Debugger pointer to NULL after it is free’d with platform_free.

Along with this checking for a NULL struct at https://github.com/nanoframework/nf-interpreter/blob/136a95eca192cf9ab881da1840025c855ab91551/src/CLR/Messaging/Messaging.cpp#L394 is required.

Expected behaviour

A clean soft reboot without hard faults is expected for a smooth operation.

Additional context

Fix suggested by Patrick Haldi from CSAEngineeringAG in CSAEngineeringAG/2g-nf-interpreter@94320db

[josesimoes]

@patrick-haldi feel free to shoot the PR when you're ready! 😉